BRITTNEY Jade Dwyer will serve at least 21 years behind bars for the stabbing murder of her grandfather in a failed robbery that was influenced by a horror TV show.

Supreme Court Justice Kevin Nicholson sentenced the 20-year-old to life imprisonment with a non-parole period of 20.5 years for the murder of her 81-year-old grandfather Robert Whitwell at his Craigmore home in Adelaide's north in August last year.

She also received an additional six months for the home invasion - bringing her total minimum imprisonment to 21 years.

Her friend, Bernadette Burns, was sentenced to life imprisonment with a non-parole period of 13.5 years for the murder as she was found not to have murderous intent.